Necessity of Ground Wire in Overhead Transmission Lines

A transmission system should be equipped with an effective protection system to prevent direct strikes of lightning strokes on the phase conductors. The protection system should also create a path over insulated structures for conducting strokes current to the earth without affecting the conductors.

One of the effective methods of providing protection of overhead transmission lines against direct lightning strokes is through overhead ground wires as shown in the below figure. The ground wire is made up of galvanized steel wire having good mechanical strength.

The ground wires are always placed above and in parallel to the line conductors. Therefore, the length of the ground wire and line conductor should always be the same.

The ground wires are always placed as the topmost conductor in high voltage transmission lines because of the following reasons,

• It acts as a protection to the power line conductors from direct lightning strokes as it is placed above the power line conductors, all the direct lightning strokes are first interrupted by ground wire and line conductors are protected.
• It provides multiple parallel paths for the lightning strokes through tower footing resistance connected. By passing through the multiple paths, the voltage of the surge gets reduced, thereby reducing the potential at the top of the tower.
• The chances of insulation failure are greatly reduced with the use of ground wires.
• If the ground wires are placed above the line conductors maintaining some distance, the chances of a short circuit between the ground wire and line conductor can be minimized.
• The location of the ground wire is decided according to the protective angle. Good shielding of the line conductor is obtained when the protection angle between the slant line connecting the ground wire and the vertical line taken as reference through a ground wire is 30° as shown below.

Requirements of Ground Wire :

The following are the requirements of a ground wire for protecting transmission lines or power conductors against lighting strokes,

• Ground wires having sufficient mechanical strength must be situated as the topmost conductor so as to shield the power conductors from direct strokes. The ground wire is made up of galvanized steel wire, which has good mechanical strength.
• The ground wires are always placed above and in parallel to the line conductors. Therefore the length of the ground wire and line conductor should always be the same.
• There must be adequate clearance between the phase conductors and ground wires, so as to ignore the flashover between them. By using a suitable length cross arm such that when a string is given a swing of 30° towards the tower structure the air gap between conductor and tower structure should be good enough to withstand the switching voltage expected on the system.
• The tower footings should have a low resistance so as to allow the lightning discharge currents to flow to the earth easily.